The phosphorylation of human and canine M-protein (having a molecular weight of 165,000 daltons) is reported. Phosphorylation was carried out using two different preparations: 1) a crude preparation of actomyosin and 2) a preparation of M-protein (according to the method of Masaki and Takaiti). Both preparations appear to contain a kinase that incorporates P32 from gamma-labeled AT32P into M-protein.